The present invention relates to a method for performing a quartz-enhanced photoacoustic spectroscopy of a gas by using a light source configured to introduce a laser beam having at least one wavelength into the gas such that at least one molecule within in the gas is stimulated generating an acoustic signal, accumulating the acoustic signal in a resonant acoustic detector and generating a resonant absorption signal relative to the gas concentration by at least one tuning fork serving as resonant acoustic detector. The invention further relates to a photoacoustic gas detector for performing a quartz-enhanced photoacoustic spectroscopy with appropriate processing means for providing a signal relative to the presence or concentration of the gas.
Such a method and gas detector are disclosed in US 2005/0117155 A1, which is incorporated in this description by reference.
In general there are three basic concepts for a photoacoustic gas sensor:
Non-Resonant Photoacoustic Gas Sensor
This concept uses a quasi-closed gas absorption cell and most often a thermal light source to measure gases in the mid-infrared. Chopping frequencies are limited by the thermal light source to no more than 100-150 Hz. The volume of the gas absorption cell is not relevant and can be minimized; the limit being contributions of dead volume in the microphone etc.
Resonant Photoacoustic Gas Sensor
For this concept, the light source is chopped at the resonant frequency of the gas absorption cell. The kHz range of cells in the cm range requires lasers as light sources, and the gas absorption cell can be open in the places of the nodes of the resonance (the microphone being placed at the anti-nodes). The advantage of such a concept is the very high rejection of ambient noise due to the high chopping frequency (ambient contributions decrease by 1/f). A limitation for this concept is the diffusion time for fast applications (typically 10-30 s).
Quartz-Enhanced Photoacoustic Gas Sensor
This concept as disclosed in US 2005/0117155 A1 replaces the resonant gas absorption cell by a resonant microphone, and the broadband microphone by a broadband volume. The absorption volume is not at all enclosed so that the sound wave generated by the gas absorption is very weak. This is counterbalanced by the extremely high quality factor of the resonant microphone. The typical approach is to use the quartz fork of a quartz wrist-watch as the resonant microphone (resonance frequency of 32,768 Hz) and the space between the prongs of the quartz fork as the absorption volume. In such a setup, the laser beam is simply focused between the prongs of the quartz fork, and the latter acts as the microphone. Apart from an extremely high diffusion rate and a minimum size (the fork is about 10 mm long and 3 mm wide), such a device is extremely cheap as quartz tuning forks are a basic component of quartz watches and produced in high quantities.
The resonant modes of a tuning fork enable an extremely high noise rejection: The sound wave of the gas absorption triggers a symmetrical vibration mode, whereas any external noise triggers an anti-symmetrical vibration mode. As both modes have slightly different frequencies with both high quality factors, the anti-symmetrical contribution is not picked up by the signal electronics.
In a photoacoustic measurement setup, a chopped light beam of a selected wavelength is fed into a gas absorption cell or volume, where its absorption by the targeted gas creates an acoustic sound wave. This sound wave is picked up by a microphone, the microphone signal being essentially proportional to the gas concentration within the gas absorption cell.
As the microphone signal is at the same time essentially proportional to the intensity of light within the gas absorption cell, a second detecting device (often an infrared detector or a photodiode) as reference signal is used to monitor the intensity of the light source over time.
In all designs of a photoacoustic gas detector, the light beam will eventually hit the structure of the gas absorption cell. The partly absorption of the light incident onto the structure creates a sound wave within the structure, this second sound wave also being picked up by the microphone at the same frequency as the gas concentration signal.
This so-called “wall noise” creates therefore an unwanted offset contribution to the microphone, and this contribution is especially important when the light source is a laser beam. Much design effort has been put in place over the last years in order to limit the contribution of the wall noise to the gas concentration measurement.